In modern, industrialized abattoirs the animals for slaughter are stunned prior to slaughtering or blood extraction. A good stunning method for the purpose of killing the animal must, from the animal protection and meat hygiene standpoint, bring the animal as rapidly and effectively as possible in to a state in which it loses consciousness and any pain sensation.
Various methods have been used in the past for stunning animals for slaughter. Electric, carbon dioxide and volt firing stunning methods have been widely used. Of late stunning methods are also known, in which the animals for slaughter are exposed to a partial air vacuum, as a result of which stunning or death occurs.
All stunning methods have a negative effect on the meat quality. In the case of stunning by current surges the PSE (pale, soft exudative) or DFD (dark, firm, dry) meat proportion is increased. In addition, there are often bone fractures and muscle bleeding. In addition, high voltages in electric stunning appear to reinforce the spasm tendency. Economic losses in particular result from bleeding, particularly of the shoulder.
Although carbon dioxide stunning does not cause muscle bleeding, petechial bleeding and bone fractures, the meat quality is generally reduced. Thus, e.g. compared with other stunning methods, the catecholamine values in the blood are at the highest levels. If e.g. halothane-positive pigs (proportion in the Federal German Republic up to 60%, in Switzerland below 10%) are stunned with carbon dioxide, then the meat quality is unacceptable and the PSE meat proportion is greatly increased. Both carbon dioxide and partial vacuum stunning methods cause the animals to be agitated for several seconds before stunning occurs.
Towards the end of the last century the first skull-penetrating stunning instruments were introduced, which are based on the principle of direct brain injury. Serious disadvantages of the earliest instruments were removed by the development of the first bolt firing apparatus between 1914 and 1920. This operating principle has been retained, with a few modifications, up to the present day. A bolt is driven by a high velocity propellant charge roughly 10 cm deep into the skull and is then removed. Thus, the bolt does not leave the firing apparatus. In addition, of late bolt firing apparatuses have become known, which are driven with compressed air. In connection therewith additional air enters the brain substance, which increases the injuries. These apparatuses have such serious problems, that they have been unusable in practise. In particular the active energy is too low, so that the stunning effect is inadequate and often subsequent shots are required. In addition, bleeding occurs in the lungs, heart and liver.
A further known skull-penetrating stunning method is shot impact stunning, which has similar disadvantages to bolt firing stunning, together with further inadequacies. For example, in Switzerland this stunning method is not authorized.
An important disadvantage of the bolt firing apparatuses in connection with industrial use is that only a limited number of animals can be shot per unit of time, so that this method is only used for large animals. Bolt firing apparatuses are only used in the case of small-scale slaughtering of pigs. In addition, the bolt firing stunning method leads to an increased proportion of light, watery meat. The bolt firing method is also problematical because the animals for slaughter and in particular pigs exhibit a terrified behaviour. This makes aiming and firing accuracy difficult, so that stunning is unreliable.
All conventional stunning methods have serious disadvantages and compromises.